The present invention relates to a method and means of construction of a galvanic cell used for the detection and measurement of small quantities of oxygen which are absorbed by the cell.
Heretofore, galvanic cells have been used to measure small amounts of oxygen introduced to the cell in gaseous form. The cell includes an anode and a cathode separated by a non-conductive, porous material that retains electrolyte and is sometimes called the diaphragm. The assembly is dampened with an electrolyte so that a precise amount of the electrolyte is absorbed in the diaphragm and then the assembly is placed in a sealed oxygen-free envelope. Some of the material suggested for use as the anode include cadmium, arsenic, bismuth, antimony, lead and ferrous hydrate. Some of the materials suggested for use as the cathode include silver, gold, platinum, copper, iridium and carbon. When a small amount of oxygen is introduced into the envelope, it is aborbed by the cathode and becomes ionized. The ions of oxygen migrate through the electrolyte to the anode and oxidize the anode material giving up electrons to the anode and these electrons flow through an external circuit path that is provided. Generally, the lower the external circuit path resistance, the faster will be the action of the cell. After the anode material becomes largely oxidized, the cell loses efficiency and its action slows down. The cell can be revitalized by applying a current to it (plus to the cathode and minus to the anode) and at the same time venting the cell to carry off the evolved oxygen.
In the past, the cell sensitivity to even very minute amounts of oxygen was improved using a porous cathode in combination with very carefully controlled amounts of electrolytes. More particularly, a minimal quantity of electrolyte was used so that the cathode did not become inundated. However, too little electrolyte increases the internal resistance of the cell and, therefore, reduces sensitivity and slows response. Hence, greater cell sensitivity was obtained only by carefully controlling the amount of elctrolyte and the porosity of the diaphragm.
The applicant has discovered that materials of which the anode is made and the materials of which the cathode is made must be carefully selected to insure that parasitic chemical reactions do not occur which would produce by-products that might tend to short circuit the diaphragm. It has been a problem in the past that undesirable materials have been used which produce such chemical reactions and by-products. In addition, some combinations of materials produce undesirable thermal electromotive forces and galvanic potentials. In this respect, copper, silver and iridium are undesirable materials.